Power generation systems often include a power converter that is configured to convert an input power into a suitable power for application to a load, such as a generator, motor, electrical grid, or other suitable load. For instance, a power generation system, such as a wind turbine system, may include a power converter for converting variable frequency alternating current power generated at the generator into alternating current power at a grid frequency (e.g. 50 Hz or 60 Hz) for application to a utility grid. An exemplary power generation system may generate AC power using a wind-driven doubly fed induction generator (DFIG). A power converter can regulate the flow of electrical power between the DFIG and the grid.
Under certain conditions (e.g., transient power conditions), a high power mismatch between the rotor and the grid connection temporally exists and voltage transients become amplified such that a DC link voltage level can increase above normal allowed or rated levels. To absorb or deflect power during such excessive power level conditions, known systems utilize a fast acting shorting means, such as a crowbar circuit, between the rotor terminals of the DFIG and the rotor converter. In operation, these shorting devices provide a short circuit at the rotor terminals to prevent excess power from flowing to the rotor converter.
For example, FIGS. 1 and 2 illustrate schematic diagrams of a conventional DFIG system 10 including a power converter 12 and a crowbar circuit 14. Specifically, FIG. 1 illustrates a single phase of the power converter 12 and FIG. 2 illustrates the three-phase connection of the crowbar circuit 14. As shown, the power converter 12 is coupled to a rotor 16 of the DFIG (not shown). The power converter 12 is a two-stage converter including both a rotor side converter 18 and a line side converter 20 coupled together by a DC link 22. Each converter 18, 20 includes a bridge circuit 24 for each phase, with each bridge circuit 24 including a plurality of switching elements (e.g., a pair of IGBTs 26 coupled in series). The power converter 12 may also include an inductive element 28 coupled in series with the bridge line of each bridge circuit 24 of the rotor side converter 18.
As particularly shown in FIG. 2, the crowbar circuit 14 is implemented using crowbar contactors 30 connected across the rotor 16 of the DFIG. Specifically, the contactors 30 are connected line-to-line such that the inductive elements 28 are coupled between the contactors 30 and the bridge circuits 24 of the rotor side converter 18. As is generally understood, the contactors 30 are configured to be normally closed so that the rotor 16 is shorted until it is verified normal power levels exist within the system 10. Upon verification of normal power levels, the contactors 30 are opened to allow power to flow to the rotor side converter 18.
As the power levels of DFIG systems have been increased over time, it has become necessary to connect the bridge circuits of the rotor side converter in parallel. For example, FIG. 3 illustrates the DFIG system 10 shown in FIG. 1 with the rotor side converter 18 being configured as a parallel bridge converter. As shown in FIG. 3, the rotor side converter 18 includes a first bridge circuit 24a and a second bridge circuit 24b coupled in parallel for each phase, with each bridge circuit 24a, 24b including a plurality of switching elements (e.g., a pair of IGBTs 26 coupled in series). Additionally, each bridge circuit 24a, 24b has a bridge line coupled to the crowbar circuit 14 via an inductive element 28. Similar to that described above with reference to FIGS. 1 and 2, the crowbar circuit 14 is typically implemented with crowbar contactors coupled line-to-line between the inductive elements 28 and the rotor 16 of the DFIG.
In addition to connecting the bridge circuits of the rotor side converter in parallel, the increase in the power levels of DFIG systems has also made it necessary to utilize larger shorting contactors that are rated to operate at higher currents. As a result, the overall cost of power converters has been increased. Moreover, it is often the case that shorting contactors large enough to handle the increased currents are unavailable.
Accordingly, it is desirable to provide a power generation system, such as a DFIG system, that includes contactors connected in parallel, thereby alleviating the need for larger, more expensive contactors. Furthermore, since parallel contactors may lead to current imbalances, it is desirable for the power converter to include a suitable means for balancing the current within the parallel contactors.